1. Field of the Invention
This invention relates to a wheel consisting of a tire, a tube, a rim, spikes, nipples and a hub for a bicycle and the like, and more particularly, to a multiplex spoke having a wire rod especially prepared from tow fiber reinforced plastics.
2. Description of the Related Art
Not only the weight of a rider and a load, but also the shocks produced while a bicycle is ridden on the ground, are applied to the wheels thereof and, particularly when the tire is punctured, a large impact is directly applied to the rim. Even in such a case, the wheel must be rigid and strong enough to be free from plastic deformation and damage for safety reasons.
As the wheel is an annulus having a relatively large radius, the angular moment of inertia is large, which requires considerable effort if the bicycle speed is to be abruptly changed while it is traveling. For this reason, there is a need in sporting or long distance bicycles, which are often subjected to sharp changes in travel speed, to have wheels which are particularly lightweight.
In order to overcome the above problems and to produce strong and lightweight wheels, there have been continuous efforts to strengthen, and reduce the weight of bicycle wheels since they were developed in the 1870's.
Although metals such as mild steel, stainless steel and titanium alloy have been used to make spokes for wheels, there is a limit to the reduction in the weight of the spoke when its strength is taken into consideration. Naturally, attempts have been made to utilize fiber reinforced plastics (hereinafter simply called the "FRP") for weight reducing purposes. However, because the FRP is generally easily affected by a local shear load, it is not applicable to the spoke portion where the spoke is screwed in the hub or the rim, to couple them together.
To solve this problem, Japanese Laid-open patent publication (Kokai) No. 51-135040 "Spoke for Wheel" discloses a method for using the FRP in the wire rod portion and metal fittings respectively at both ends of a spoke. The method is characterized by providing a junction of a spoke and a rim incorporating a rim nut and, when they are assembled, inserting the head metal fitting into the spoke hole in the hub first and then turning the rim nut to obtain spoke tension. However, the shear rigidity coefficient of the FRP formed by uniformly drawing fibers in the longitudinal direction is roughly 500 kg/mm.sup.2 at most, and the wire rod is twisted, transformed and wrenched off by screwing torque before a tension of 80.about.100 kg is normally obtained. Thus the aforesaid arrangement cannot be put to practical use.